An image capture device includes an image sensor and an imaging lens. The imaging lens focuses light onto the image sensor to form an image, and the image sensor converts the light into electric signals. The electric signals are output from the image capture device to other components of a host electronic system. The electronic system may be, for example, a mobile phone, a computer, a digital camera or a medical device.
As pixel cells become smaller, it becomes more difficult for the pixel cell to output a signal of adequate strength that can be easily deciphered by downstream signal processing. Moreover, there are demands on the image sensor to perform over a large range of lighting conditions, varying from low light conditions to bright light conditions. This performance capability is generally referred to as having high dynamic range imaging (HDRI or alternatively just HDR). Thus, prior art solutions for decreasing the size of the pixel cell limit the dynamic range of the pixel cell.
FIG. 1 is a diagram of a prior art four-transistor (4T) pixel cell included within an image sensor array. Pixel cell 100 includes light sensing element (i.e., photodiode) 101, transfer transistor 102, reset transistor 103, source-follower transistor 104 and row select transistor 105.
During operation, transfer transistor 102 receives transfer signal TX, which transfers the charge accumulated in photodiode 101 to floating diffusion node 106. Reset transistor 103 is coupled between power rail VDD and floating diffusion node 106 to reset pixel cell 100 (e.g., discharge or charge floating diffusion node 106 and photodiode 101 to a preset voltage) under control of reset signal RST.
Floating diffusion node 106 is coupled to control the gate terminal of source-follower transistor 104. Source-follower transistor 104 is coupled between power rail VDD and row select transistor 105. Row select transistor 105 selectively couples the output of pixel circuitry to the readout column 190 under control of row select signal RS.
In normal operation, photodiode 101 and floating diffusion node 106 are reset by temporarily asserting the reset signal RST and transfer signal TX. The accumulating window (i.e., exposure period) is commenced by de-asserting the transfer signal TX and permitting incident light to charge photodiode 101. As photo-generated electrons accumulate on photodiode 101, its voltage decreases (electrons are negative charge carriers). The voltage or charge on photodiode 101 is indicative of the intensity of the light incident on photodiode 101 during the exposure period. At the end of the exposure period, the reset signal RST is de-asserted to isolate floating diffusion node 106 and transfer signal TX is asserted to couple photodiode 101 to floating diffusion node 106. The charge transfer causes the voltage of floating diffusion node 106 to drop by an amount proportional to photo-generated electrons accumulated on photodiode 101 during the exposure period.
Floating diffusion node 106 is designed to be relatively small in order to achieve high transfer or conversion gain; however, in high illumination conditions, the amount of charge (signal) produced by photodiode 101 may be greater than the capacity of floating diffusion node 106. This will result in saturation of the floating diffusion node, thereby producing a reduced dynamic range, as well as reduced signal-to-noise ratio (SNR).
Descriptions of certain details and implementations follow, including a description of the figures, which may depict some or all of the embodiments described below, as well as discussing other potential embodiments or implementations of the inventive concepts presented herein. An overview of embodiments of the invention is provided below, followed by a more detailed description with reference to the drawings.